1. Field of the Invention
The present invention relates to a developing device and an image forming unit used for electrophotographic image formation, and an image forming apparatus provided with the same.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copier, a printer or a facsimile machine includes a developing device for forming a toner image on an image bearing member by supplying a developer to an electrostatic latent image formed on the image bearing member (e.g. photoconductive drum or transfer belt) and developing the electrostatic latent image.
The developing device includes, as main constituent elements, a storage portion for storing a developer including nonmagnetic toner particles and magnetic carrier particles while agitating them, a magnetic roller forbearing a developer layer (so-called magnetic brush layer) by receiving the developer from the storage portion, and a developing roller for receiving and bearing the toner particles from the magnetic brush layer on a bearing surface based on a potential difference between the developing roller and the magnetic roller and then supplying the toner particles to the image bearing member based on a potential difference between the developing roller and the image bearing member.
In the developing device constructed as described above, an attached matter in the form of a thin film is likely to be formed on the bearing surface of the developing roller due to the long-term use of the developing device and a continuous printing operation at a high coverage rate. The attached matter is toner particles, external additive of the toner particles and the like. If an attached matter is formed on the bearing surface, the resistance of the bearing surface increases and electric charges are accumulated on the bearing surface. Thus, an apparent voltage increases. This reduces the potential difference between the developing roller and the magnetic roller and the developing roller cannot sufficiently receive the toner particles from the magnetic roller. As a result, there is a problem of reducing the density of a toner image formed on the image bearing member.
As a countermeasure against the above problem, the attached matter on the bearing surface of the developing roller needs to be removed to suppress a reduction in the density of the toner image. In a conventional developing device, the rotating speed of a magnetic roller is made slower or a distance between a developing roller and the magnetic roller is made shorter when a developing operation is not performed than when the developing operation is performed, thereby strongly bringing a magnetic brush layer on the magnetic roller into contact with an attached matter to remove the attached matter.
In the conventional developing device, no special member is employed to remove the attached matter, but it is difficult to remove the attached matter only by changing the rotating speed of the magnetic roller or the distance between the developing roller and the magnetic roller particularly when the thickness of the attached matter is large.